Fuel cell electrolyte control

ABSTRACT

An electrolyte control system for a fuel cell which has an anode and a cathode spaced by an electrolyte carrying matrix wherein porous capillary conduits communicate from an electrolyte reservoir to points uniformly along the matrix. For example, the porous capillary conduits can be positioned uniformly along the matrix adjacent the cathode and thereby supply electrolyte directly to the cathode, and to the anode and matrix at whatever rate the matrix will absorb the electrolyte.

United States Patent Dallas, Tex.

Appl. No. F iled, Patented Assignee FUEL CELL ELECTROLYTE CONTROL 2Claims, 3 Drawing Figs.

u.s. Cl I 136/86 Int. Cl ....ll0lm 27/20 Field of Search 136/86References Cited UNITED STATES PATENTS 1/1968 Kordesch 3,458,357 7/1969Truitt 3,466,197 9/1969 Bawa Primary Examiner-Winston A. DouglasAssistant ExaminerH. A. F eeley Attorneys-Samuel M. Mims, .Ir., James 0.Dixon, Andrew M. l-Iassell, Harold Levine, Melvin Sharp and Richards,Harris & Hubbard ABSTRACT: An electrolyte control system for a fuel cellwhich has an anode and a cathode spaced by an electrolyte carryingmatrix wherein porous capillary conduits communicate from an electrolytereservoir to points uniformly along the matrix. For example, the porouscapillary conduits can be positioned uniformly along the matrix adjacentthe cathode and thereby supply electrolyte directly to the cathode, andto the anode and matrix at whatever rate the matrix will absorb theelectrolyte.

FUEL CELL ELECTROLYTE CONTROL This invention relates to fuel cells. Inanother aspect, this invention relates to the control of electrolyteflow to the electrodes of fuel cells.

A conventional fuel cell configuration includes a pair of porous,conductive electrodes spaced apart and contacted by an electrolyte whichis carried by a matrix of a dielectric material, which provides amultiplicity of pores. In the operation of this cell, a suitablereactant gas is passed on one side of each electrode and contactselectrolyte in the porous structure of each electrode to provide forcell reaction. In cells of this type the inert matrix is in directcontact with a reservoir of electrolyte, and the electrolyte is fed by awicklike action the matrix and the electrodes.

The present trend in fuel cell development is toward lighter, smaller,and thinner fuel cells. The trend has resulted in the development ofclosely spaced anodes and cathodes, and therefore in very thin butuniform electrolyte-containing matrices. Thus, it is necessary that thethin matrices utilized in these cells be uniform in thickness andporosity so that a substantially uniform contact of electrolyte is madewith the surface of the electrodes.

One method of forming the thin matrices is initially to form moldingcomposition of matrix material and then apply this material unifonnlyaround the surface of an electrode, for example, the anode. The cathodeis then placed against the exposed surface of the matrix to form a fuelcell unit. Electrolyte can be added to the matrix by depositingelectrolyte on the exposed matrix area between the electrodes. Theelectrolyte will then wick through and saturate the thin matrix betweenthe electrodes. While the older conventional matrix material comprised amultiplicity of finely divided ceramic material such as magnesium oxideparticles, lithium aluminate particles or aluminum oxide particles, therecently developed thin-walled matrix material includes, in addition tothe ceramic particles, binders and reaction products such as alkalimetal aluminates and silicates.

Problems have sometimes occurred when using molten carbonate electrolytefuel cells with this newly developed matrix material, particularly inconjunction with the use of anodes with compound cross-sectionalconfigurations, such as corrugations, folds, or a multiplicity ofcylindricallike members which are fastened together. It has been foundthat when these particular anodes are used in conjunction with the newlydeveloped matrix material, the molten carbonate electrolyte willfrequently wick more rapidly through the anodes than it will through thematrix. This action will result in flooding of the anode and cause asubstantial decrease in efficiency of the fuel cell. This floodinggenerally occurs because electrolyte will only wick a short distancethrough the matrix before substantial reaction products betweenmaterials in the newly developed matrix, such as silicates, will reactwith the electrolyte and form reaction products which in turn plug thepores of the matrix. After this, the electrolyte will wick around theplugged areas directly to the anode. The capillaries formed by the foldsand/or creases of the anode will cause the electrolyte to then wick upthe anode until it is saturated.

Therefore, one object of this invention is to provide a method ofimproving the efficiency of fuel cells in which the electrolytecontacting the electrodes is carried by a thin solid porous matrixmaterial.

Another object of this invention is to provide a novel system forcontrolling the flow of proper amounts of electrolyte to electrodes infuel cells.

According to this invention electrolyte is supplied to the electrodes ofa fuel cell which has at least one porous anode and at least one porouscathode separated by an electrolytecarrying matrix, by the action ofpermeable capillary conduit means which communicate from an electrolytesupply uniformly along the length of the matrix between the electrodes.Thus, the capillary conduit means of this invention function totransport electrolyte along the length of the matrix between theelectrodes and are provided with permeable walls which will deliverelectrolyte uniformly to the matrix.

Preferably the permeable capillary conduit means will communicateuniformly along the length of the matrix adjacent the cathode. Theelectrolyte delivered by the permeable capillary conduits will therebywet the cathode and uniformly supply electrolyte to the matrix atwhatever rate the matrix will absorb the electrolyte, thus preventingflooding of the anode but assuring uniform contact of the anode by theelectrolyte at points where the anode contacts the matrix.

According to a preferred embodiment of this invention a pair of screenswhich are fastened together and held a fixed distance apart to assuregood capillary action are positioned between the cathode and the inertmatrix. The screens communicate with an electrolyte supply and therebyallow electrolyte to be uniformly distributed to the matrix and theelectrodes of the fuel cell as described above. Even more preferably, asecond pair of these screens which communicate with the electrolytesupply are positioned along the other side of the cathode to assure thatthe cathode is uniformly saturated with electrolyte.

This invention can be more easily understood from a study of thedrawings in which:

FIG. 1 is a perspective view of a fuel cell unit which includes theelectrolyte control system of this invention;

FIG. 2 is a sectional view along lines 2-2 of FIG. 1;

FIG. 3 is a sectional view along lines 3-3 of FIG. 1.

Now referring to FIGS. 1 and 2 fuel cell unit 10 generally comprises aboxlike structure containing four parallel connected fuel cells. It isto be understood that any number of fuel cells connected in paralleland/or in series can be utilized with this invention. Thus, the numberof fuel cells in unit 10 together with the electrical interconnectionstherebetween is not intended to limit the scope of this invention.

Enclosures 11 and 12 which carry cavities l3 and 14, respectively, areconnected at opposite ends of fuel cell unit 10. Fuel inlet conduit 15communicates through enclosure 11 and has annular electric terminal 16operatively attached thereto. In similar manner, fuel outlet conduit 17communicates through enclosure 12 and has annular electric terminal 18operatively connected thereto. Electrical conductive wires lead fromterminal 16 and terminal 18 to a suitable circuit.

Now referring to FIGS. 2 and 3, the internal structure of fuel cell unit10 is illustrated in detail. FIG. 2 illustrates the cathode structure,with the anodes omitted for purposes of clarity and convenience. In FIG.3 the anode structure is illustrated, with the cathodes omitted. Anodesl9 and 20 generally comprise a single sheet-metal screen formed into aseries of teardrop-shaped folds. The teardrop-shaped folds are heldtogether by braising or spot welding points of contact such asillustrated at points 21. This folded pattern allows reactant ful tofreely flow through spaces 22. Alternatively, a variety of otherelectrode structures can be utilized for electrodes 19 and 20. Forexample, a coarse wire mesh similar to a kitchen scour pad can be used,or, a number of cylindrical or tubularshaped pieces stacked one onanother can be used. Electrodes l9 and 20 can be made from any suitableanode material, for example, to mesh nickel screen.

Anodes l9 and 20 carry matrices 23 and 24 molded around the outsideperiphery thereof as illustrated in FIG. 3. These matrices can be madefrom any suitable moldable matrix composition known in the art. Suitablesuch compositions are disclosed in copending US. Pat. application Ser.No. 601,782, filed Dec. 14, I966. For example, the matrix material canbe prepared by mixing powdered sodium lithium carbonate, aluminum oxide,magnesium oxide, and silicon oxide (SiO in a dry state, heating themixture in an air atmosphere for about 4 hours at about 800 C., andregrinding the resulting sintered material back to a powder. Thesintering operation causes the starting materials to partially react,thereby resulting in a mixture of unreacted sodium lithium carbonate,aluminum oxide, and magnesium oxide with the reaction products, lithiumaluminate, lithium silicate, sodium aluminate, and sodium silicate. Aportion of this sintered material is then mixed with sodium silicate ndpowdered aluminum, zinc, and chromium oxide. Sufficient water is thenadded to the mixture to make a slurry for ease of application toelectrodes 19 and 20. The slurry is then applied around electrodes 19and 20 and allowed to dry and harden into rigid but yet porouselectrolyte matrix bodies.

Cathodes 25, 26, and 27 are identical and generally comprise corrugatedmember welded between two side sheet members. Electrodes 25, 26 and 27serve as the cathodes for fuel cell unit and can be made from anysuitable porous material known in the art such as 80 to 150 mesh silverplated stainless steel screen, for example. As illustrated in thedrawings, every electrode in unit 10 except electrodes 25 and 27 servesas a dual electrode for two cells.

Electrolyte-permeable capillary conduits 28 are positioned on both sidesof electrodes 25, 26, and 27. Electrolyte-permeable capillary conduits28 comprise a sheet of screen material 29 which has been welded to theside sheet members of corrugated electrodes 25, 26, and 27 at points 30by a suitable method, such as roll welding. Weld points 30 can be spacedbetween the two screens as desired to obtain good wicking of theelectrolyte, for example, on l/ l6 to V4 inch centers. In this preferredembodiment, screens 29 comprise a corrosion-resistant material, such asa corrosion-resistant stainless steel screen. It is generally preferredto use from 80 to 150 mesh stainless steel screens as screen members 29.As shown in F IG. 3, screen members 39 which are positioned adjacentmatrix 23 and matrix 24 extend around the sides of channels 31, and arewelded thereto at points 32. The upper ends 33 of screen members 29which are positioned on the outside edges of electrodes 25 and 27 extenda short distance above the top of electrodes 25 and 27, respectively.Thus, when unit 10 is impregnated with electrolyte by depositing asuitable electrolyte such as molten carbonate on top of unit 10 withinrails 31 and along extensions 33, the electrolyte will immediately wickthrough capillary conduits 28 and become evenly distributed along thelength of matrices 23 and 24 between the anodes and cathodes.Additionally, the electrolyte will uniformly wet the cathodes 25, 26,and 27.

The capillary pore size carried by permeable capillary conduits 28 canbe varied by techniques known in the art to satisfy the wicking andcapillary characteristics of any particular electrolyte. For example,consider the equation: 'y=% hgdr, where 7 equals surface tension of aliquid; h equals height of the column of the liquid above the lowerliquid level; g equals acceleration due to gravity; d equals density ofthe liquid; and r equals radius of the capillary pore. By rearrangementof the equation, it can be seen that the capillary pore radius isdirectly proportional to the surface tension of the liquid and inverselyproportional to the height of the column, the gravitational accelerationand the liquid density. Consequently, with proper sizing the desiredamount of capillarity can be obtained for a given electrolyte. Forexample, with the capillary screens illustrated in the drawing thedistance between the screens can vary from 0.01-0.005 inch and yieldgood results with a sodium-lithium carbonate molten electrolyte.

It must be noted that the number of capillary conduits 28 adjacentelectrodes 25, 26, and 27 is not intended to limit the scope of thisinvention. For example, capillary conduits 28 need only be placedadjacent electrodes 25, 26, and 27 at points between the electrodes andmatrices. In some operations it may be desired to place capillaryconduit 28 within the matrix between the electrodes. However, accordingto a preferred embodiment of this invention, permeable capillaryconduits 28 are placed on both sides of electrodes 25, 26, and 27. Thisarrangement will allow electrolyte to flow through electrolyte permeablecapillary conduits 28, uniformly wet electrodes 25, 26, and 27 (thecathodes) and uniformly distribute electrolyte to the surface ofmatrices 23 and 24. The electrolyte will then wick evenly through porousmatrices 23 and 24 by capillary action and contact the curvedteardropshaped convolutions of electrodes 19 and 20 which are positionedadjacent the matrices. Makeup electrolyte will be continuously fed tothe matrices during long term cell operation at whatever rate thematrices will absorb the electrolyte.

Now referring to FIG. 2, the electrical connections between theelectrodes and electrical terminals 16 and 18 are illustrated in detail.Conductive end plate 34 is connected such as by welding with conductiveenclosure 11. Slots 35 within conductive end plate 34 communicate fromchamber 13 to spaces 22 within electrodes 19 and 20. Flanges 35 whichare operatively connected to electrodes 19 and 20 are connected insimilar manner such as by welding to conductive end plate 34. It isnoted that electrodes 25, 26, and 27 are spaced from and therebyinsulated from conductive end plate 34. Conductive end plate 36 isconnected such as by welding to conductive enclosure 12 and therebycommunicates with electric terminal 18. Slots 37 communicate betweenchamber 14 and spaces 22 of electrodes 19 and 20. Electrodes 25, 26, and27 are operatively connected such as by welding to conductive end plate36. Electrodes l9 and 20 are spaced from conductive end plate 36.

In operation, fuel cell unit 10 is placed within a suitable environmentwherein an oxidizer reactant will continuously pass through electrodes25, 26, and 27. Electrodes 19 and 20 are provided with a suitable fuelvia inlet conduit 15 and chamber 14. Fuel cell unit 10 will functionwith a variety of reactants, but the preferred system is a fuel feedcomprising hydrogen and an oxidizer mixture comprising oxygen and carbondioxide. The hydrogen can be either pure or mixed along with variousother gases such as nitrogen, carbon dioxide, carbon monoxide, lighthydrocarbons, water vapor, and the like. The oxygen can be either pureor supplied as air. This particular system can effectively use variousalkali metal carbonates as electrolytes. A preferred electrolyte is aeutectic mixture of sodium carbonate and lithium carbonate, e.g., 50percent molar sodium carbonate and 50 percent molar lithium carbonatehaving a melting point of about 500 C. Fuel cell unit 10 is placedwithin a suitable heating device which is supplied with the oxidizeratmosphere and maintained at a temperature in the vicinity of 600 C.,e.g., about 650 C. Merely placing fuel cell unit 10 within a ceramicwall oven (or other insulated casing means) which is provided with agaseous flow of oxygen and carbon dioxide, or air and carbon dioxide, ina direction parallel to electrodes 25, 26, and 27, will suffice.

Matrices 23 and 24 are impregnated with electrolyte such as for example,by pouring the molten carbonate eutectic upon channels 31 and screenextensions 33. The carbonate electrolyte will wick through permeablecapillary conduits 28 in a manner described above to thereby wetelectrodes 25, 26, and 27, and matrices 23 and 24.

The hydrogen feed gas is then passed through inlet conduit 15, intochamber 13 and through slots 35, spaces 22 of electrodes l9 and 20, tochamber 14. Since the capillary action of capillary conduits 28 hasuniformly distributed the electrolyte to the surface of matrices 23 and24 between adjacent electrodes, the electrolyte will wick through thematrices and wet electrodes 19 and 20 only at the points where theycontact the matrices. Thus, a triple interface within spaces 22 willoccur and the hydrogen gas will diffuse through the nickel screen to theliquid electrolyte interface. Since the nickel screens serve not only asan electrode and a porous interface between fuel and electrolyte, butalso as a catalyst to promote the half cell oxidation reaction, greaterefficiency of the anode will result than in conventional cells whereinthe anode is flooded or substantially covered with thick liquidelectrolyte film. The reaction occurring in fuel electrodes 19 and 20(the anodes) is as follows:

g co H O+CO l 2e It must be understood that the particular screen-typecapillary conduits illustrated in the drawings are not intended to limitthe scope of this invention. Any suitable type electrolyte permeablecapillary conduits can be used in the practice of this invention. Forexample, a series of porous capillary tubes, twisted strands, or a sheetof woven material can be used as the permeable capillary conduits of theinvention. When using a sheet of woven material it is generallypreferred that relatively larger strands be woven in the lengthwisedirection to relatively smaller strands in the crosswise direction sothat the relatively larger strands will communicate from the electrolytesupply to the matrix. Also, the cathode can be constructed with a seriesof porous wicking tubes as an integral part thereof, and for example,silver plating can be applied over the entire surface without seriouslyaffecting the wicking of the electrolyte by the capillary tubes.

Additionally, in the shorter term fuel cell operations wherein makeupelectrolyte is not essential, it is only necessary to have the permeablecapillary conduits communicating with the matrix for the relativelyshort period of time that is required for the matrix to become wettedwith electrolyte. In these operations, the capillary conduits used inthis invention can be made of a material which will react with theelectrolyte once it has been delivered to the matrix to form an oxidewhich is nondeleterious to fuel cell operation. Examples of these typesof capillaries include zirconia tape or a woven cloth made from highalumina fibers. It is only necessary that the cloth or tape chosen notreact so rapidly as to form a dam or blockage of the capillary beforethe matrix is saturated with electrolyte.

While this invention has been described in reference to its preferredembodiments, it is to be understood that various modifications withinthe scope of the appended claims will now be apparent to one skilled inthe art upon reading the specification.

I claim:

1. A fuel cell unit comprising:

a. at least one porous anode spaced from at least one porous cathode bya porous electrolyte support matrix.

Said anode is a sheet of electrically conductive material which isfolded into a teardrop pattern, a portion of which is positioned withinsaid porous matrix, the other portion providing passages for a reactant;

b. means to supply a reactant to said passages of gaseous to said anodeand means to supply a gaseous reactant to said cathode;

c. electrolyte supply means; and

d. permeable capillary conduit means operatively communicating betweensaid electrolyte supply means uniformly to points only said porouselectrolyte support matrix between said porous electrolyte supportmatrix and said cathode.

2. The fuel cell of claim 1 wherein said permeable capillary conduitmeans comprises a pair of screen members operatively fastened togetherto fonn capillary pores.

2. The fuel cell of claim 1 wherein said permeable capillary conduitmeans comprises a pair of screen members operatively fastened togetherto form capillary pores.